Functional monomer impurity affects adhesive performance

Objective The functional monomer 10-MDP has been considered as one of the best performing functional monomers for dental adhesives. Different adhesives containing 10-MDP are commercially available, among which many so-called 'universal' adhesives. We hypothesize that the quality of the functional monomer 10-MDP in terms of purity may affect bonding performance. Methods We therefore characterized three different 10-MDP versions (10-MDP-KN provided by Kuraray Noritake; 10-MDP-PCM provided by PCM; 10-MDP-DMI provided by DMI) using NMR, and analyzed their ability to form 10-MDP-Ca salts on dentin using XRD. The 'immediate' and 'aged' micro-tensile bond strength (μTBS) to dentin of three experimental 10-MDP primers was measured. The resultant interfacial adhesive-dentin ultra-structure was characterized using TEM. Results NMR disclosed impurities and the presence of 10-MDP dimer in 10-MDP-PCM and 10-MDP-DMI. 10-MDP-PCM and 10-MDP-DMI appeared also sensitive to hydrolysis. 10-MDP-KN, on the contrary, contained less impurities and dimer, and did not undergo hydrolysis. XRD revealed more intense 10-MDP-Ca salt deposition on dentin induced by 10-MDP-KN. The adhesive based on the experimental 10-MDP-KN primer resulted in a significantly higher 'immediate' bond strength that remained stable upon aging; the μTBS of the experimental 10-MDP-PCM and 10-MDP-DMI adhesives significantly dropped upon aging. TEM revealed thicker hybridization and more intense nano-layering for 10-MDP-KN. Significance It was concluded that primer impurities and the presence of 10-MDP dimer affected not only hybridization, but also reduced the formation of 10-MDP-Ca salts and nano-layering. 10-MDP in a high purity grade is essential to achieve durable bonding.